1. Field of the Invention
This invention relates to a sensor for various types of electrochemical measurement and a method which uses the sensor to measure quantitatively the corrosion protective properties of a coating film on metal or of a rust film created on the surface of steel materials for the purpose of diagnosing the degree of degradation of the coating film and the corrosion protective properties of the rust film.
2. Description of Related Art
In electrochemical measurement wherein an electrode such as a counter electrode or a reference electrode is used to measure potential or current/ voltage characteristics at the surface of a metal material to be measured or impedance at the metal surface under the application of a small AC current for the purpose of measuring the degree of corrosion in the metal surface or the corrosion rate, an electrolyte dissolved in either water or organic solvent or molten salt is used as liquid electrolyte. In any case, the electrochemical measurement using liquid must be carried out in a container such as beaker or cell and has applicability limited to measurement conducted in laboratories.
Therefore, a sensor capable of permitting electrochemical measurement of the surface of metal such as steel materials used in existing structures directly in the field has been proposed by JP-A-62-229056 or the corresponding U.S. Pat. No. 4,806,849 issued to Kihira et al on Feb. 21, 1989 and entitled "Method and Apparatus for Diagnosing Degradation of Coating Film on Metal Material". In the proposal, a cylindrical chamber made of silicon rubber has an open mouth portion in which a sponge impregnated with a liquid electrolyte is filled and when conducting measurement, the open mouth portion is brought into intimate contact with the surface of a metal material to be measured so that the sealing function of the open mouth portion may eliminate a gap through which the liquid electrolyte leaks.
Further, considering that during measurement the liquid electrolyte remains, in great amounts, on the object being measured and evaporation and leakage of the liquid takes place through the sealing portion, the proposed sensor has conveniently an additional liquid electrolyte supplementing apparatus. Moreover, since in diagnosis of an actual structure, non-uniformity of degradation of coating film is found from one location to another, the aforementioned U.S. Patent also proposes a method, by which a great number of points are measured to determine a statistical distribution and a two-dimensional distribution of impedance of the coating film for the purpose of diagnosing quantitatively the degree of degradation of the coating film, as well as simultaneous measurement for efficient implementation of the method by using a plurality of sensors.
On the other hand, there is available a steel material as represented by weathering steel which utilizes a rust film created in natural environment so as to be improved in corrosion resistance. The corrosion protective properties of such a rust film can be diagnosed quantitatively by means of an apparatus as disclosed in JP-A-60-100751 which measures and evaluates AC impedance of the rust film by using two sensors, like the sensor of the aforementioned U.S. Patent, holding a liquid electrolyte.
The electrochemical measurement using the electrolyte in liquid state is convenient for measurement of a sample placed in a container such as beaker but is inconvenient for direct measurement of part of an actual structure in the field because sealing for prevention of leakage of liquid is needed and particularly, it is conducted only at the cost of very degraded efficiency when there are many measurement points.
The conventional sensor for diagnosing the degree of degradation of a coating film on metal faces first of all a problem that the liquid electrolyte tends to leak and adhere to a portion outside the open mouth portion of the sensor and electric current leaks through the liquid electrolyte remaining at the portion not being under measurement, and a second problem that bubbles remaining in the open mouth portion sometimes cause inequality between the area of the open mouth portion and the actual contacting area and it is rather difficult to mount the sensor without causing variations in contacting area.
Further, to cope with gradual drying of the sponge accommodated in the sensor, the sensor of the aforementioned U.S. Patent employs the additional liquid electrolyte supplementing apparatus which transports the liquid electrolyte under pressure, whereby a proper amount of liquid electrolyte can be supplemented to control wettability and the liquid electrolyte can sufficiently penetrate into the gap at the interface between the surface to be measured and the sensor.
However, when the area of the conventional open mouth portion is increased larger than 1.2 cm.sup.2, a liquid electrolyte supplementing apparatus of larger capacity is required and in addition bubbles tend to remain at the interface to make the contacting state unstable resulting in variation of effective contact area, while when the area is decreased smaller than 1.2 cm.sup.2, the surface tension creates a droplet which is larger than the area of the open mouth portion and a very small area can not be measured.
When multi-point simultaneous measurement is desired to be conducted by arranging many sensors on an object being measured at a time, many liquid electrolyte supplementing apparatus, identical in number to the sensors, are needed and checking each sensor for its sealing condition and the presence or absence of bubbles at the interface is very laborious and time-consuming.
Incidentally, coated metal is used in a variety of fields of social capital and industrial capital including civil engineering and construction such as bridges, roofs, wall members, tanks, piping and steel towers and transportation equipments such as ships, trains, automobiles and containers. For these structures, coating specifications complying with their purposes are available to offer various kinds of coating films ranging from coating film of relatively low impedance to that of very high impedance.
However, the measurement typically uses a constant pulse current which is small enough not to damage coated films and such a small pulse current generator limits the maximum measurable impedance to about 200 megohms (M ohm). Accordingly, measurable objects are limited and the measurement can be applied to only a painting system of relatively low impedance in which an oil paint is used for the top coating and a red lead paint is used for the ground coating.
In coated metal used for structures, the coated film is considered to be electrically equivalent to a parallel circuit of resistances within the range in which the coated film is considered to be uniform from the standpoint of macroscopic impedance measurement and hence the resistance value measurable by the conventional impedance measuring method increases with increasing of the area of the open mouth of the sensor which is in a range of 1 to 100 cm.sup.2.
Therefore, the application of the conventional impedance measuring method would be advantageously extended to a coating system using coat of higher impedance by increasing the area of open mouth of the sensor in the above range. In the conventional method, however, compatibility between the increase in the area of the object to be measured and the steady contacting state of the sensor as well as unimpairment of ease of handling is difficult to achieve.